Yeah it's gotta be something else stripping your nutrients.
Things that can lower your phosphate are carbon dosing, using phosphate reducing media, water changes (not including topping off with RO), or growth of corals/macroalgae. Any of these things happen recently? There is a chemical process that binds phosphate to substrate but I wouldn't assume that is your problem. If you have a really small tank, I suppose it's possible that topping off with a gallon of water diluted the phosphate to undetectable levels.
That could be it then. Try to limit the fluctuation in water volume as it makes all of your parameters shift in ways that hurt coral. If you minimize those fluctuations with daily top offs or an ato you can maintain that stability. Keeping it stable over time will give you more accurate test results too so you feel more comfortable about when parameters really need to be adjusted.
Good luck going forward. Small tanks can be tricky that way; with such low water volume small changes have a more dramatic effect on the inhabitants. A week of evaporation will raise your salinity and mess with your parameters more significantly than it would in a 75 gallon for example. In addition to keeping the water level maintained, I would recommend testing the salinity of any water you use for changes prior to putting it in if you aren't already doing so. That'll prevent an accidental swing in parameters as well if the salinity is matched.
